The present invention relates generally to telemetry systems utilized in conjunction with subterranean wells and, in an embodiment described herein, more particularly provides a synchronous CDMA telemetry system for use in a wellbore.
There exists a need for a method of transmitting data at very high rates in subterranean wells. One example of such a need is that of transmitting very high data rate seismic data from geophones positioned in a wellbore. In this example, a data rate of 30 Mbps might be required.
Unfortunately, a data rate of 30 Mbps is far greater than the capabilities of present telemetry systems. For example, conventional wireline logging telemetry systems are capable of only about 300 Kbps, which is two orders of magnitude less than the desired data rate.
Furthermore, conventional wireline logging telemetry systems are not designed to transmit data from widely distributed data sources. Instead, the data sources used with wireline logging tools are typically closely arranged in a well. Transmission of data from widely distributed data sources, on the other hand, can cause delays in relative transmission times between the respective data sources, thereby altering the relationships between the transmitted data.
In the case of seismic data, it is important for evaluation of the data that the data from separate sensors measured at a particular time be transmitted in a manner which permits this time relationship to be preserved. One manner of furthering this goal is to transmit the data from separate sources simultaneously via the same transmission channel. However, transmission of data from separate sources via the same transmission channel presents other challenges, such as how to differentiate one source""s data from another source""s data.
In carrying out the principles of the present invention, in accordance with an embodiment thereof, a method is provided which solves the above problem in the art.
In one aspect of the invention, the method includes the steps of installing multiple sensors in a wellbore, multiplying each data bit of each sensor""s output by a digital code and simultaneously transmitting the encoded sensor outputs to a remote location via a transmission channel. Each of the sensors is included in a sensor node and produces a respective digital output. In an example of the method, the sensors are geophones widely distributed in a wellbore, and the sensor nodes are all connected to a cable serving as the transmission channel.
In another aspect of the invention, the codes used to encode the sensor outputs are unique to each of the sensors and are orthogonal with respect to each other. Preferably, the codes are walesh codes. In this manner, the encoded outputs may be transmitted simultaneously on the same transmission channel without interference or cross-talk between the transmissions.
In yet another aspect of the invention, the encoded sensor outputs are modulated on a carrier frequency prior to being summed on the transmission channel. When the resulting signal is received at the remote location, it is demodulated. The demodulated signal is then separately multiplied by each of the codes to produce respective decoded transmissions containing separate contributions to the signal. These separate decoded transmissions are then integrated over the length of each data bit to produce an output from which the original data may be interpreted.
In still another aspect of the invention, the transmissions from the various sensor nodes are synchronized. A phase lock loop is used to phase lock the transmissions. A sliding correlator is used to obtain synchronization by adjusting an offset timer of each sensor node. An early-late correlator is used to maintain synchronization.
These and other features, advantages, benefits and objects of the present invention will become apparent to one of ordinary skill in the art upon careful consideration of the detailed description of a representative embodiment of the invention hereinbelow and the accompanying drawings.